High-density polyethylene which is used in wide applications such as films, pipes, and bottle containers, has been conventionally prepared by using a Ziegler-Natta catalyst or a chromium catalyst. However, because of the nature of such catalysts, there has been limitation on the control of the molecular weight distribution or composition distribution of the polymer.
In recent years, several methods have been disclosed for preparation of an ethylene polymer having excellent moldability and mechanical strength, including an ethylene homopolymer or an ethylene/α-olefin copolymer of relatively small molecular weights and an ethylene homopolymer or an ethylene/α-olefin copolymer of a relatively large molecular weight, according to a continuous polymerization technique, using a single-site catalyst which facilitates the control of the composition distribution.
JP-A No. 11-106432 discloses a composition prepared by melt-blending a low molecular weight polyethylene with a high molecular weight ethylene/α-olefin copolymer, which is obtained by polymerization using a supported, geometric constraint type single-site catalyst (CGC/Borate-based catalyst). However, it is expected that sufficient mechanical strength would not be exhibited in the case of the carbon number being less than 6 according to the method disclosed in the above-mentioned patent application. Further, because the molecular weight distribution (Mw/Mn) of the single-stage polymerization product is broad, it is also expected that the mechanical properties of the product, such as impact strength, would be insufficient, as compared with the single-stage product having a narrower molecular weight distribution.
WO 01/25328 discloses an ethylene polymer which is obtained by solution polymerization in the presence of a catalyst system comprising CpTiNP(tBu)3Cl2 and borate or alumoxane. This ethylene polymer has a weak crystalline structure due to the presence of branches in the low molecular weight component, and thus it is expected that the polymer has poor mechanical strength.
EP 1201711 A1 discloses an ethylene polymer which is obtained by slurry polymerization in the presence of a catalyst system comprising ethylene bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride and methylalumoxane supported on silica. Among these ethylene polymers, a single-stage polymerization product has a wide molecular weight distribution (Mw/Mn), and thus it is expected that it would have insufficient impact strength and the like, as compared with a single-stage product of a narrower molecular weight distribution. Further, it is inferred that a broad molecular weight distribution means heterogeneity of the active species, and consequently there is a concern that the composition distribution broadens, thereby resulting in deterioration of long lifetime properties such as environmental stress cracking resistance (ESCR).
JP-A No. 2002-53615 discloses an ethylene polymer which is obtained by slurry polymerization using a catalyst system comprising methylalumoxane and a zirconium compound having a specific salicylaldimine ligand supported on silica. Although the patent application does not disclose the preferred range of the carbon number of α-olefin that is to be copolymerized with ethylene, in regard to the ethylene polymer obtained from 1-butene (number of carbon atoms=4) which is used as the α-olefin in Examples of the patent application, the carbon number envisaged to be too small to exhibit a sufficient mechanical strength.
The ethylene (co)polymer prepared using a Ziegler catalyst as described in Japanese Patent No. 821037, or the like has methyl branches in the molecular chain as a result of side production of methyl branches during the polymerization. It was found that these methyl branches were embedded in the crystal, thus weakening the crystal, and this caused deterioration of mechanical strength of the ethylene (co)polymer. Further, in regard to the copolymer of ethylene and an α-olefin, when the copolymer contained almost no α-olefin, a hard but brittle component was produced, while when an excessive amount of α-olefin was subject to copolymerization, a soft component with weak crystalline structure was produced, and thus it may cause tackiness due to broad composition distribution. Moreover, since the molecular weight distribution was broad, there were problems such as the phenomenon that a low molecular weight polymer adheres onto the surface of a molded product as a powdery substance, and so on.
The ethylene polymer that is obtained by polymerization using a metallocene catalyst as described in JP-A No. 9-183816, or the like causes side production of methyl branches during the polymerization, thereby lowering the mechanical strength.
An ethylene polymer which is obtained by polymerization using a chromium catalyst exhibits small extension of molecular chain because of the presence of a long chain branch, and thus has poor mechanical strength and long lifetime properties such as environmental stress cracking resistance (ESCR). Further, as a result of side production of a methyl branch during the polymerization, there exist methyl branch groups in the molecular chain. This has been a cause for lowering the mechanical strength.
The ethylene polymer which is obtained by polymerization using a constrained geometry catalyst (CGC) as described in WO 93/08221, or the like has methyl branches in the molecular chain, as a result of side production of a methyl branch during the polymerization. These methyl branches are embedded in the crystals, and thus weaken the crystalline structure. This has been a cause for lowering the mechanical strength. Further, the molecular extension of molecular chain was small because of the presence of long chain branches, and thus the mechanical strength and long lifetime properties such as environmental stress cracking resistance (ESCR) were insufficient.
An ethylene polymer which is obtained by high pressure radical polymerization has methyl branches or long chain branches in the molecular chain, as a result of side production of methyl branches or long chain branches during polymerization. These methyl branches are embedded in the crystals, thereby weakening the crystalline strength. This has been a cause for lowering the mechanical strength. Further, the presence of long chain branches resulted in small extension of molecular chain as well as a broad molecular weight distribution, and thus the long lifetime properties such as environmental stress cracking resistance (ESCR) were poor.